The present invention relates to electrolytic cells comprising polymeric composition electrode and electrolyte members and to a method of economically making such cells. In particular, the invention relates to rechargeable lithium battery cells comprising an electrode-intermediate polymeric separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for making such cells in which the ion source electrode is a lithium compound or other material capable of intercalating lithium ions, and where an inter-electrode membrane comprises a plasticized polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on porous separator structures or membranes which physically entrained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric were saturated with solutions of a lithium compound, such as LiClO.sub.4, LiPF.sub.6, or LiBF.sub.4, in an organic solvent, e.g., propylene carbonate, diethoxyethane, or dimethyl carbonate, to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes provided the necessary Li.sup.+ ion mobility for conductivities in the range of about 10.sup.-3 S/cm.
Subsequent developments, such as described in U.S. Pat. No. 5,296,318 have provided electrolytic battery cells which have both positive and negative electrodes comprising compounds capable of intercalating ions and include strong, non-porous, flexible polymeric electrolytic cell separator membrane materials which contain lithium salt electrolyte solutions and remain functional over temperatures ranging well below room temperature. These electrolyte membranes are employed either as separator elements with mechanically assembled battery cell components or in composite battery cells constructed of successively coated layers of electrode and electrolyte compositions. In each of these implementations, however, the polymeric electrolyte/separator elements often contain the lithium electrolyte salts at the time of cell assembly and, due to the hygroscopic nature of those salts, necessitate extraordinary environmental conditions during cell assembly.
More recent developments have provided a manner of utilizing these improved polymeric electrolyte membrane and electrode compositions which substantially eliminates the need for special environmental controls during cell manufacture. Typically, the polymeric electrode and electrolyte/separator layers are thermally bonded to form a laminated cell structure which ensures optimum interlayer reactivity and enables the postponement of sensitive electrolyte incorporation until the final stages of battery construction or even later in its application as an activating fluid.
The laminated layer structure of these cells also provides a ready means for incorporating electrical current collector elements, usually as additional outer conductive layers or foils which can add further strength to the cell assembly. In order to provide optimum access of activating electrolyte solution to the electrode and separator layers, it is preferred that at least one of these outer collector layers, when comprising a normally impermeable material such as metal foil, be of an open grid or mesh structure, perforated, or otherwise similarly formed to allow fluid permeation.
Batteries of various size, capacity, and voltage range can readily be fashioned from the layered cell structure by overlaying a number of cells or manifolding a single cell of extended dimension. Although manifolding is useful in its economy of operations and ability to provide directly, i.e, without additional insulating elements, a proper arrangement of respective electrode collectors, the folding of a perforate or grid collector tends to result in the stress fracture or rupture of that weaker element which may ultimately lead to a significant interruption of current flow to a battery terminal. The present form of battery construction provides a means for alleviating such stresses and, additionally, simplifies the production of battery packages in a variety of sizes, capacities, and voltages.